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Viscoelastic properties of the interphase in fibre reinforced polymers - measurement and simulation
(1999)
Mit der Gruppenstrahlertechnik der Bundesanstalt für
Materialforschung und -prüfung sind umfangreiche Untersuchungen an einer
plattierten Druckbehälter-Testwand durchgeführt worden. In der vorliegenden
Testwand mit einer Wanddicke von 149 mm befinden sich künstliche
Testreflektoren im Schweißnaht- und Plattierungsbereich, die mit Gruppenstrahler-
Prüfköpfen mit unterschiedlichen Frequenzen, Einschallwinkeln und
Einschallrichtungen nachgewiesen und in ihrem Anzeigenverhalten analysiert
wurden.
Zur Rekonstruktion der Reflektoranzeigen wurde ein SAFT - Algorithmus
verwendet, der die Variation der Einschallwinkel berücksichtigt. Zum Vergleich
sind die Reflektoren auch mit der TOFD - Technik analysiert worden. Ziel der
Untersuchungen war der direkte Vergleich der Eignung der SAFT- und TOFDTechnik
zur quantitativen Reflektorgrößenbestimmung.
An exciting challenge is to create unduloid-reinforcing fibers with tailored dimensions to produce synthetic composites with improved toughness and increased ductility. Continuous carbon fibers, the state-of-the-art reinforcement for structural composites, were modified via controlled laser irradiation to result in expanded outwardly tapered regions, as well as fibers with Q-tip (cotton-bud) end shapes. A pulsed laser treatment was used to introduce damage at the single carbon fiber level, creating expanded regions at predetermined points along the lengths of continuous carbon fibers, while maintaining much of their stiffness. The range of produced shapes was quantified and correlated to single fiber tensile properties. Mapped Raman spectroscopy was used to elucidate the local compositional and structural changes. Irradiation conditions were adjusted to create a swollen weakened region, such that fiber failure occurred in the laser treated Region producing two fiber ends with outwardly tapered ends. Loading the tapered fibers allows for viscoelastic energy dissipation during fiber pull-out by enhanced friction as the fibers plough through a matrix. In these tapered fibers, diameters were locally increased up to 53%, forming outward taper angles of up to 1.8°. The tensile strength and strain to failure of the modified fibers were significantly reduced, by 75% and 55%, respectively, ensuring localization of the break in the expanded region; however, the fiber stiffness was only reduced by 17%. Using harsher irradiation conditions, carbon fibers were completely cut, resulting in cottonbud fiber end shapes. Single fiber pull-out tests performed using these fibers revealed a 6.75-fold increase in work of pull-out compared to pristine carbon fibers. Controlled laser irradiation is a route to modify the shape of continuous carbon fibers along their lengths, as well as to cut them into controlled lengths leaving tapered or cotton-bud shapes.
The present study aims at the development of photodegradable epoxy based materials comprising o-nitrobenzyl ester links that undergo well defined bond cleavage in response to UV irradiation. New bi-functional epoxy based monomers bearing o-nitrobenzyl ester groups are synthesized and thermally cured with an anhydride hardener to yield photosensitive polymers and duromers. The UV induced changes in solubility are exploited for the preparation of positive-type photoresists. Thin patterned films are obtained by photolithographic processes and characterized by microscopic techniques. The results evidence that sensitive resist materials with good resolution and high contrast behavior can be accomplished. Along with resist technology, the applicability of o-nitrobenzyl chemistry in the design of recyclable polymer materials with thicknesses in the millimeter range is evaluated. By monitoring the thermo-mechanical properties upon UV illumination, a distinctive depletion of storage modulus and glass transition temperature is observed with increasing exposure dose. Additionally, single fiber pull-out tests are carried out revealing a significant decrease of the interfacial adhesion at the fiber-matrix interface due to the phototriggered cleavage reaction.